1. Field of the Invention
The present invention relates to an improved operation of electrolytic cells for electrolyzing aqueous alkali metal halide brines in electrolytic cells employing a permselective ion-exchange membrane to separate the anolyte chamber from the catholyte chamber.
2. Definition Used Herein
Herein both "carbon oxide" and "carbonate" are defined to mean: carbon dioxide; or carbonic acid; or a carbonate or a bicarbonate of an alkali metal or an alkaline earth metal (including magnesium); or combinations thereof. These definitions are useful because of the shifting nature of these elements from one to another with shifts in pH of the brine. For example, the carbonate ion shifts to carbon dioxide when the pH is lowered.
3. Discussion of Prior Art
The electrolytic production of chlorine and caustic by the electrolysis of brine has been well known for many years. Historically, diaphragm cells using hydraulically-permeable, asbestos diaphragms which being vacuum-deposited onto foraminous steel cathodes have been widely commercialized. Such diaphragm cells produce NaCl-containing NaOH catholytes, because NaCl passes through the hydraulically-permeable diaphragm from the anolyte to the catholyte. Such NaCl-containing caustic is generally of low caustic concentration and requires a de-salting process and extensive evaporation of water to obtain a low-salt, high concentration caustic for industrial purposes.
In permeable diaphragm-type cells and in mercury-type cells, it has long been known to treat the brine before it is electrolyzed to improve these cells long-term performances. This treatment is done to reduce the hardness cations, and herein is referred to a "conventional" brine treatment. In "conventional" brine treatment sodium carbonate and sodium hydroxide are added to the brine primarily to form calcium carbonate and magnesium hydroxide as insoluble compounds. Most of the solids are then settled and the remainder filtered in this conventional treatment to produce brine having a residual hardness value of only from about 1 to 5 ppm (expressed as ppm calcium). To achieve these low hardness values, an excess of sodium carbonate and caustic are used in the treatment to produce treated brine having a pH of from about 10 to 12 and containing from about 0.4 to 1.5 grams per liter of "carbonate" or "carbon oxide" (expressed as grams/liter of sodium carbonate.)
In any event this conventional brine treatment has not proved sufficient in conjunction with the use of the relatively new permselective cation exchange membranes.
In recent years, the chlor-alkali industry has focused much of its attention on developing membrane cells to produce low-salt or salt-free, high concentration caustic in order to improve quality and avoid the costly desalting and evaporation processes. Membranes have been developed for that purpose which are substantially hydraulically-impermeable, but which will permit hydrated sodium ions to be transported from the anolyte portion to the catholyte portions of the electrolysis cell, while substantially preventing transport of chloride ions. Representative of such membranes are the perfluorosulfonic acid membranes made and sold by the E. I. duPont de Nemours & Co., Inc., under the tradename, Nafion, and the perfluorocarboxylic acid membranes of the Asahi Glass Industry Co., Ltd., of Tokyo, Japan. See U.S. Pat. No. 4,065,366 to Oda et al for a description of the latter carboxylic acid type membranes.
Such membrane-type cells are operated by flowing a brine solution into the anolyte portion and by providing salt-free water to the catholyte portion to serve as the caustic medium during electrolysis. The anodic reactions and cathodic reactions are the same regardless of whether a membrane cell or a diaphragm cell is employed.
It is known that, with the passage of time, the operating voltage of such membrane type cells gradually increases. Of course, this increasing voltage has meant an increase in electric power used by the cells. Eventually the electrical energy consumed by the cell per unit of product produced rises to the point of being so costly that the cells have to be shut down and the membranes washed or replaced. This, of course, is very expensive, and, hence, means have been sought to slow down, if not, stop this rise of cell voltage and electric energy consumption, so as to extend the useful life of these expensive membranes, to lengthen the time between expensive cell shutdowns, and to operate the cells at reduced energy consumption.
Polyvalent cations remaining in the brine cell feed even after conventional treatment have been identified by the prior art as being the principal source of these problems; particularly the polyvalent cations often referred to as the "hardness"; e.g., the calcium and magnesium cations (Ca.sup.++ and Mg.sup.++), present in the brine feed..sup.1 FNT .sup.1 See U.S. Pat. No. 3,793,163 to R. S. Dotson (1974); The Asahi Chemical Membrane Chlor-Alkali Process, page 5 of a paper presented by Maomi Seko of Asahi Chemical Industry Co., Ltd., of Tokyo, Japan, at The Chlorine Institute, Inc., 20th Chlorine Managers Seminar, at New Orleans, Louisiana on Feb. 3, 1977; Effect of Brine Purity on Chlor-Alkali Membrane Cell Performance, a paper originally presented by Charles J. Molnar of E. I. duPont de Nemours & Co., Inc., and Martin M Dorio of Diamond Shamrock Corporation at The Electrochemical Society Fall Meeting held October, 1977, at Atlanta, Georgia; The Commerical Use of Membrane Cells in Chlorine/Caustic Plants, pages 6-9 of a paper presented by Dale R. Pulver of Diamond Shamrock Corporation at The Chlorine Institute's 21st Plant Manager's Seminar, at Houston, Texas, on Feb. 15, 1978; Nafion.RTM. Membranes Structured for High Efficiency Chlor-Alkali Cells, a paper presented by Charles J. Hora of Diamond Shamrock Corporation and Daniel E. Maloney of E. I. duPont de Nemours & Co., Inc., at The Electrochemical Society Fall Meeting, October, 1977, Atlanta, Georgia; U.S. Pat. No. 4,115,218 to Michael Krumpeit (1978); U.S. Pat. No. 4,073,706 to Zoltan Nagy (1978); U.S. Pat. No. 3,988,223 to S. T. Hirozawa (1976); U.S. Pat. No. 4,204,921 to W. E. Britton et al (1980); U.S. Pat. No. 4,202,743 to Oda et al (1980); and U.S. Pat. No. 4,108,742 to Seko et al (1978).
As indicated above, it is well known that multivalent cations such as calcium, magnesium, iron and aluminum form insoluble precipitates in and on the membrane causing an escalation of cell voltage and in extreme cases, a decrease in cell current efficiency. Thus, Hora and Maloney (Nafion.RTM. Membranes Structured for High Efficiency Chlor-Alkali Cells, The Electrochemical Society Fall Meeting, October 1977, Altanta, Georgia) report that the use of pure brine is especially important when operating any membrane cell. The use of ion exchange treatment of the brine feed results in hardness levels less than one ppm.
Molnar and Dorio (Effects of Brine Purity on Chlor-Alkali Membrane Cell Performance, same meeting as above) studied the specific effects of Ca and Mg on chlor-alkali membrane cell performance. In this paper, twenty ppm of Ca added to the brine resulted in an immediate decrease in current efficiency and increase in cell voltage. The same level of Mg resulted in a decrease in current efficiency, but it caused little if any effect on voltage over a short period (20 days). Addition of only 6 ppm Ca, when using a membrane having sulfonamide functional groups, resulted in an immediate descrease in current efficiency and increase in voltage.
Seko (in a paper presented at The Asahi Chemical Membrane Chlor-Alkali Process, The Chlorine Institute, 20th Chlorine Plant Managers Seminar, New Orleans, Louisiana, February 1977) reported that, while using membranes having carboxylic acid functional groups, it is most important to purify the brine to maintain as low a level as possible of multivalent cations. To accomplish this, conventionally treated brine was further purified by ion exchange treatment. Further treatment of this conventionally treated brine with chelating type ion-exchange resins to remove more hardness has no effect on the carbonate and caustic level. When the brine is fed to chlor-alkali cells, the carbonate, in the acid conditions of the anolyte compartment, forms carbon dioxide and leaves the cell mixed with the chlorine gas. The excess carbonate produces from about 0.2 to 0.6% CO.sub.2 by volume in the chlorine depending on the amount of excess carbonate used (see pg. 4, D. Pulver, The Commercial Use of Membrane Cells in Chlorine/Caustic Plants, Chlorine Plant Manager's Seminar Proceedings, 21st Meeting, Houston, Texas, February 1978).
Oda, et al (U.S. Pat. No. 4,202,743) when using a carboxylic acid functional membrane and producing between 20 and 45% caustic in the catholyte compartment have reported the concentration of calcium should be less than 0.08 mg/liter (0.066 ppm) and the concentration of magnesium less than 0.04 mg/liter in the cell feed brine.
Dotson (U.S. Pat. No. 3,793,163) reported that the addition of a compound capable of forming, at an elevated pH, an insoluble reversible gel with polyvalent cations was beneficial to the operation of electrolytic cells employing permselective cation-exchange membranes. The additive compounds are selected from the group consisting of free acid and alkali metal phosphates, orthophosphates and metaphosphates. The additives result in current and voltage efficiencies remaining at optimum high levels during extended periods of operation since unsequestered polyvalent cations do not reach the membrane.
It would be advantageous to obtain as good or better results to the solutions of these problems as do the above described methods without having to go to the extremes to which they go in removing, or tying up, the calcium cations. The present invention does just this.